How to emulate a do-while loop?
PythonWhile LoopDo WhilePython Problem Overview
I need to emulate a do-while loop in a Python program. Unfortunately, the following straightforward code does not work:
list_of_ints = [ 1, 2, 3 ]
iterator = list_of_ints.__iter__()
element = None
while True:
if element:
print element
try:
element = iterator.next()
except StopIteration:
break
print "done"
Instead of "1,2,3,done", it prints the following output:
[stdout:]1
[stdout:]2
[stdout:]3
None['Traceback (most recent call last):
', ' File "test_python.py", line 8, in <module>
s = i.next()
', 'StopIteration
']
What can I do in order to catch the 'stop iteration' exception and break a while loop properly?
An example of why such a thing may be needed is shown below as pseudocode.
State machine:
s = ""
while True :
if state is STATE_CODE :
if "//" in s :
tokens.add( TOKEN_COMMENT, s.split( "//" )[1] )
state = STATE_COMMENT
else :
tokens.add( TOKEN_CODE, s )
if state is STATE_COMMENT :
if "//" in s :
tokens.append( TOKEN_COMMENT, s.split( "//" )[1] )
else
state = STATE_CODE
# Re-evaluate same line
continue
try :
s = i.next()
except StopIteration :
break
Python Solutions
Solution 1 - Python
I am not sure what you are trying to do. You can implement a do-while loop like this:
while True:
stuff()
if fail_condition:
break
Or:
stuff()
while not fail_condition:
stuff()
What are you doing trying to use a do while loop to print the stuff in the list? Why not just use:
for i in l:
print i
print "done"
Update:
So do you have a list of lines? And you want to keep iterating through it? How about:
for s in l:
while True:
stuff()
# use a "break" instead of s = i.next()
Does that seem like something close to what you would want? With your code example, it would be:
for s in some_list:
while True:
if state is STATE_CODE:
if "//" in s:
tokens.add( TOKEN_COMMENT, s.split( "//" )[1] )
state = STATE_COMMENT
else :
tokens.add( TOKEN_CODE, s )
if state is STATE_COMMENT:
if "//" in s:
tokens.append( TOKEN_COMMENT, s.split( "//" )[1] )
break # get next s
else:
state = STATE_CODE
# re-evaluate same line
# continues automatically
Solution 2 - Python
Here's a very simple way to emulate a do-while loop:
condition = True
while condition:
# loop body here
condition = test_loop_condition()
# end of loop
The key features of a do-while loop are that the loop body always executes at least once, and that the condition is evaluated at the bottom of the loop body. The control structure show here accomplishes both of these with no need for exceptions or break statements. It does introduce one extra Boolean variable.
Solution 3 - Python
My code below might be a useful implementation, highlighting the main difference between [tag:do-while] vs [tag:while] as I understand it.
So in this one case, you always go through the loop at least once.
first_pass = True
while first_pass or condition:
first_pass = False
do_stuff()
Solution 4 - Python
do {
stuff()
} while (condition())
->
while True:
stuff()
if not condition():
break
You can do a function:
def do_while(stuff, condition):
while condition(stuff()):
pass
But
- It's ugly.
- Condition should be a function with one parameter, supposed to be filled by stuff (it's the only reason not to use the classic while loop.)
Solution 5 - Python
Exception will break the loop, so you might as well handle it outside the loop.
try:
while True:
if s:
print s
s = i.next()
except StopIteration:
pass
I guess that the problem with your code is that behaviour of break
inside except
is not defined. Generally break
goes only one level up, so e.g. break
inside try
goes directly to finally
(if it exists) an out of the try
, but not out of the loop.
Related PEP: http://www.python.org/dev/peps/pep-3136
Related question: https://stackoverflow.com/questions/653509/breaking-out-of-nested-loops
Solution 6 - Python
The way I've done this is as follows...
condition = True
while condition:
do_stuff()
condition = (<something that evaluates to True or False>)
This seems to me to be the simplistic solution, I'm surprised I haven't seen it here already. This can obviously also be inverted to
while not condition:
etc.
Solution 7 - Python
Here is a crazier solution of a different pattern -- using coroutines. The code is still very similar, but with one important difference; there are no exit conditions at all! The coroutine (chain of coroutines really) just stops when you stop feeding it with data.
def coroutine(func):
"""Coroutine decorator
Coroutines must be started, advanced to their first "yield" point,
and this decorator does this automatically.
"""
def startcr(*ar, **kw):
cr = func(*ar, **kw)
cr.next()
return cr
return startcr
@coroutine
def collector(storage):
"""Act as "sink" and collect all sent in @storage"""
while True:
storage.append((yield))
@coroutine
def state_machine(sink):
""" .send() new parts to be tokenized by the state machine,
tokens are passed on to @sink
"""
s = ""
state = STATE_CODE
while True:
if state is STATE_CODE :
if "//" in s :
sink.send((TOKEN_COMMENT, s.split( "//" )[1] ))
state = STATE_COMMENT
else :
sink.send(( TOKEN_CODE, s ))
if state is STATE_COMMENT :
if "//" in s :
sink.send(( TOKEN_COMMENT, s.split( "//" )[1] ))
else
state = STATE_CODE
# re-evaluate same line
continue
s = (yield)
tokens = []
sm = state_machine(collector(tokens))
for piece in i:
sm.send(piece)
The code above collects all tokens as tuples in tokens
and I assume there is no difference between .append()
and .add()
in the original code.
Solution 8 - Python
Python 3.8 has the answer.
It's called assignment expressions. from the documentation:
# Loop over fixed length blocks
while (block := f.read(256)) != '':
process(block)
Solution 9 - Python
I believe that this do-while simulation on python has a syntax format closest to the do-while structure format present in C and Java.
do = True
while do:
[...]
do = <condition>
Solution 10 - Python
for a do - while loop containing try statements
loop = True
while loop:
generic_stuff()
try:
questionable_stuff()
# to break from successful completion
# loop = False
except:
optional_stuff()
# to break from unsuccessful completion -
# the case referenced in the OP's question
loop = False
finally:
more_generic_stuff()
alternatively, when there's no need for the 'finally' clause
while True:
generic_stuff()
try:
questionable_stuff()
# to break from successful completion
# break
except:
optional_stuff()
# to break from unsuccessful completion -
# the case referenced in the OP's question
break
Solution 11 - Python
while condition is True:
stuff()
else:
stuff()
Solution 12 - Python
Quick hack:
def dowhile(func = None, condition = None):
if not func or not condition:
return
else:
func()
while condition():
func()
Use like so:
>>> x = 10
>>> def f():
... global x
... x = x - 1
>>> def c():
global x
return x > 0
>>> dowhile(f, c)
>>> print x
0
Solution 13 - Python
Why don't you just do
for s in l :
print s
print "done"
?
Solution 14 - Python
If you're in a scenario where you are looping while a resource is unavaliable or something similar that throws an exception, you could use something like
import time
while True:
try:
f = open('some/path', 'r')
except IOError:
print('File could not be read. Retrying in 5 seconds')
time.sleep(5)
else:
break
Solution 15 - Python
For me a typical while loop will be something like this:
xBool = True
# A counter to force a condition (eg. yCount = some integer value)
while xBool:
# set up the condition (eg. if yCount > 0):
(Do something)
yCount = yCount - 1
else:
# (condition is not met, set xBool False)
xBool = False
I could include a for..loop within the while loop as well, if situation so warrants, for looping through another set of condition.
Solution 16 - Python
You wondered:
> What can I do in order to catch the 'stop iteration' exception and break a while loop properly?
You could do it as shown below and which also makes use of the assignment expressions feature (aka “the walrus operator”) that was introduced in Python 3.8:
list_of_ints = [1, 2, 3]
iterator = iter(list_of_ints)
try:
while (element := next(iterator)):
print(element)
except StopIteration:
print("done")
Another possibility (that would work from Python 2.6 to 3.x) would be to provide a default
argument to the built-in next()
function to avoid the StopIteration
exception:
SENTINEL = object() # Unique object.
list_of_ints = [1, 2, 3]
iterator = iter(list_of_ints)
while True:
element = next(iterator, SENTINEL)
if element is SENTINEL:
break
print(element)
print("done")
Solution 17 - Python
While loop:
while condition:
logic
Do while loop:
while True:
logic
if not condition:
break
Solution 18 - Python
See if this helps :
Set a flag inside the exception handler and check it before working on the s.
flagBreak = false;
while True :
if flagBreak : break
if s :
print s
try :
s = i.next()
except StopIteration :
flagBreak = true
print "done"
Solution 19 - Python
The built-in iter function does specifically that:
for x in iter(YOUR_FN, TERM_VAL):
...
E.g. (tested in Py2 and 3):
class Easy:
X = 0
@classmethod
def com(cls):
cls.X += 1
return cls.X
for x in iter(Easy.com, 10):
print(">>>", x)
If you want to give a condition to terminate instead of a value, you always can set an equality, and require that equality to be True
.